Method of manufacturing a magnetic ferrite core



June 19, 1956 F. G. BROCKMAN METHOD OF MANUFACTURING A MAGNETIC FERRITE CORE 2 Sheets-Sheet 1 Filed April 10, 1953 Fig.|

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FRANK G. BROCKMAN AGENT June 19, 1956 F. G. BROCKMAN METHOD OF MANUFACTURING A MAGNETIC FERRITE CORE Filed April 10, 1953 2 Sheets-Sh'eet 2 nouns TEUPERATWE'U I 200 Fig.6

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FRANK G. BROCKMAN AGENT Uflitcd States Patent METHOD. OF MANUFACTURING A MAGNETIC FERRITE CORE Application April 10, 1953, Serial No. 348,005 4 Claims. (Cl. 252-625) This invention relates to soft ferromagnetic cores for magnetic induction devices and, in particular, relates to methods for manufacturing soft ferromagnetic cores con stituted by magnetic ferrites.

Ferromagnetic ferrites are compositions of'one or more oxides of certain bivalent metals and ferric oxide, i. e., materials whose compositions can be expressed as MF6204, M being one or more bivalent metals such as Cu, Ni, Zn, Mn, Mg and Cd, and mixed crystals of two or more ferrites, which have been sintered to produce a material having very high values of initial permeability. These materials have been fully described in U. S. Patents Nos. 2,452,529; 2,452,530; 2,452,531; 2,555,711; 2,579,978 and in a monograph by I. L. Snoek entitled New Developments in Ferromagnetic Materials, published in 1947.

Cores made from these ferrite materials have the advantage of a high initial permeability, a low loss factor even at extremely high frequencies, and a high electrical resistance to eddy currents. Consequently, the excellent properties exhibited by these materials make them emmiuently suitable for high-frequency applications.

It is the principal object of this invention to produce improved magnetic ferrite cores having a low or zero temperature coefficient of initial permeability over a wide range of temperatures.

A further object of this invention is to provide a method for manufacturing an improved magnetic ferrite core exhibiting a low or zero temperature coeificient of initial permeability over the temperature range from 20 to 85 C.

A still further object of the invention is to provide a technique for regulating the temperature dependence of initial permeability of magnetic ferrites.

These and further objects of the invention will be best understood from the following description.

I have discovered that the temperature dependence of initial permeability of a ferromagnetic ferrite core can be predetermined by controlling the temperature and duration of the final sintering operation by which the core is fabricated. More particularly, quite unexpectedly I have found that, contrary to all existing theories and explanations of the behaviour of ferrite cores, ferromagnetic fern'te cores having zero temperature coeflicients, and in some cases even negative temperature coefficients, could be produced by a careful adjustment of the final sintering temperature and/ or sintering time.

The consequence of this discovery is that it now becomes possible to manufacture ferromagnetic ferrite cores having, in addition to all of the heretofore known advantageous properties, low or zero temperature coefficients of initial permeability over a wide range of temperatures. Hence, cores for induction devices capable of operating at low temperatures and also at high temperatures are now made possible.

In accordance with my invention, I first mix ferriteforming constituents, e. g., FezOa and one or more metal oxides such as NiO, MnO, ZnO, MgO, CuO or metal Patented June 19, 1956 compounds which can be thermally decomposed to produce those oxides, in. theproportions such that after subsequent heating a ferrite, or mixed crystals of two or more ferrites, is formed. After I mix the ferrite-forming constituents together, I heat the mixture, preferably after a body has been pressed therefrom, at a temperature and for a time at which the ferrite formed has the temperature coefficient of initial permeability that is desired.

Generally, the ferrite-forming constituent, e. g., metal ferrite-forming oxides, are thoroughly ground and mixed together to form an intimate mixture of the constituents. A body is pressed from this mixture and heated at a temperature from about 1000" to 1400 C. for from A to 24 hours depending upon. the initial constituents and the shaping pressure. It is to be understood, of course, that the: correct choice of sintering temperature and time of sintering will depend. not only on the initial constituents and shaping pressure, but also upon thedesired temperature coeflicient. In other words, the two main factors, e. g., sintering time and. temperature, must be carefully selected, for a given core, to produce a core having a desired temperature coeflicient of initial permeability.

I have further found that the shaping presure, i. e., the pressure used to form a body, has a slight effect on the temperature coeflicient and may be employed to effect slight additional variations of the temperature coefficient of initial permeability. However, this expedient alone has no significant effect on the temperature coefficient, itseflfect being additive rather than primary. The basic factors are as stated above, the sintering temperature and duration or time of sintering.

Though the correct choice ofsintering time. and temperature will, of course, vary for different ferrites and starting constituents, these conditions are readily ascertainable byone skilled in the art once having knowledge of the principal factors affecting the temperature dependence of initial permeability. Therefore, while the invention is about. tobe described in connection with a limited number of representative examples, I wish it to be clearly understood that my invention resides. in the discovery that the temperature coelficient of initial permeability' is subject to a wide degree of control by varying'the temperature and time of sintering. My invention is therefore not to be limited by the representative ex.- amples I employ to describe the procedural detail for carrying out, the invention, but is defined by the claims forming a part of this specification.

The invention will now be, described with reference to the accompanying drawing in which:

Fig. 1 shows a graph of initial permeability (,uo) vs. temperature for three ferrite samples sintered at different temperatures;

Fig. 2 shows a graph similar to Fig. 1 for another set of ferrite samples;

Fig. 3 shows a graph of temperature coefficient vs. sintering temperature for four ferrite samples sintered for different durations;

Fig. 4 shows a graph of initial permeability 40) vs. temperature for four ferrite samples sintered at the same temperature for diiferent lengths of time;

Fig. 5 shows a graph similar to Fig. 4 for another set of ferrite samples;

Fig. 6 shows a graph of initial permeability vs. temperature for four ferrite samples shaped with different forming pressures.

All of the processing of the ferrite raw materials preparatory to the final pressing and sintering operations to form the desired shaped cores was the same for all samples, as follows: The raw oxides in the desired proportions, e. g., a 20/30/50 nickel-zinc-ferrite produced by starting with 20 mol. per cent of nickel oxide, 30 mol. per cent of zinc oxide and 50 mol. per cent of ferric oxide, were mixed in a high speed blender, e. g., a Waring Blendor, using alcohol as a fluid. After filtering and drying, the mix was presintered for one hour at 1040 C. in air. The presintered mixture was then ball milled for about 8 hours using alcohol as the milling fluid. 'After further filtering and drying, the mix was ready for the final processing in accordance with the techniques to be described in connection with each of the graphs of the accompanying drawing.

Referring to Fig. 1, three identical batches of a /30/50 nickel-Zinc-ferrite were each pressed with a forming pressure of 15,100 lbs./sq. in. and sintered for one hour in air at 1250 C., 1275 C. and 1300 C., respectively. The graphs show that at a sintering temperature of 1250 C., a nearly flat characteristic is obtained.

Fig. 2 shows the effect of different sintering temperatures for three 17/33/50 nickel-zinc-ferrite samples, all sintered for one hour in air after an initial pressing at 15,100 lbs./sq. in.

In Fig. 3, the temperature coeflicient is plotted against sintering temperature for four 20/ 30/ 50 nickel-zino-ferrite samples initially pressed at 15,100 lbs./sq. in. and sintered for 1, 2, 4 and 8 hours, respectively, in air. The temperature coefiicient is defined as where A t is the change in initial permeability taken over the temperature interval, At, from C. to 85 C. The 1.0 in the denominator is the initial permeability at 25 C. It will be noted that three of the curves will pass through zero at specific sintering temperatures.

Fig. 4 shows the eifect of a change in sintering time for four 20/ 50 nickel-zinc-ferrite samples initially pressed at 15,000 lbs/sq. in. and sintered at 1250 C. in air. The curve associated with a sintering time of one hour is practically flat.

The conditions of pressing and sintering for the samples exhibiting the characteristics shown in Fig. 5 were identical to that of the samples resulting in the curves of Fig. 4, except that the sintering temperature was reduced to 1200 C. It should be noted that the sample sintered for one hour actually exhibited a slightly negative temperature coefiicient.

Fig. 6 shows the effect on the temperature coeflicient of varying the forming pressure of four 20/ 30/50 nickelzinc-ferrite samples which were subsequently sintered at 1200 C. for 2% hours in air.

What I claim is:

1. A method of manufacturing a ferromagnetic ferrite core having a zero temperature coefiicient of permeability comprising the steps forming a mixture containing about 20 mol. per cent of nickel oxide, 30 mol. per cent of zinc oxide and 50 mol. per cent of ferric oxide, presintering said mixture at a temperature of about 1040 C. for about 1 hour, grinding and compacting the presintered mixture under pressure to form a shaped core, sintering the soshaped core at a temperature of from about 1000 C. to 1400 C. for about A to 24 hours, and discontinuing the sintering when the temperature coeflicient of permeability from about 20 C. to 85 C. reaches a value of about zero.

2. A method of manufacturing a ferromagnetic ferrite core having a zero coefficient of initial permeability which comprises the steps of forming a mixture containing about 20 mol. per cent of nickel oxide, 30 mol. per cent of zinc oxide and 50 mol. per cent of ferric oxide, presintering said mixture at a temperature of about 1040 C. for about 1 hour, grinding and compacting the presintered mixture into a shaped core at a pressure at which after sintering the core has the desired temperature coefiicient of permeability, and sintering the so formed core at 1200 C. for about 2 /2 hours to obtain thereby a core exhibiting a substantially zero temperature coefficient of permeability from about 20 C. to C.

3. A method of manufacturing a ferromagnetic ferrite core having a zero coeflicient of initial permeability which comprises the steps of forming a mixture containing about 20 mol. per cent of nickel oxide, 30 mol. per cent of zinc oxide and 50 mol. per cent of ferric oxide, presintering said mixture at a temperature of about 1040 C. for about 1 hour, grinding and compacting the presintered mixture under pressure to form a shaped core and sintering the so shaped core at 1250 C. for about 1 hour to obtain thereby a core exhibiting a substantially zero temperature coeflicient of permeability from about 20 C. to 85 C.

4. A method of manufacturing a ferromagnetic ferrite core having a zero coefficient of initial permeability which comprises the steps of forming a mixture containing about 20 mol. per cent of nickel oxide, 30 mol. per cent of zinc oxide and 50 mol. per cent of ferric oxide, presintering said mixture at a temperature of about 1040 C. for about 1 hour, grinding and compacting the presintered mixture at a pressure of about 5,000 to 7,500 lbs/sq. in. to form a shaped core and sintering the so shaped core at 1200 C. for about 2 /2 hours to obtain thereby a core exhibiting a substantially zero temperature coefficient of permeability from about 20 C. to 85 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,551,711 Snoek et al. May 8, 1951 2,579,978 Snoek et al Dec. 25, 1951 2,636,860 Snoek et a1. Apr. 28, 1953 

1. A METHOD OF MANUFACTURING A FERROMAGNETIC FERRITE CORE HAVING A ZERO TEMPERATURE COEFFICIENT OF PERMEABILITY COMPRISING THE STEPS FORMING A MIXTURE CONTAINING ABOUT 20 MOL. PER CENT OF NICKEL OXIDE, 30 MOL, PER CENT OF ZINC OXIDE AND 50 MOL. PER CENT OF FERRIC OXIDE, PRESINTERING SAID MIXTURE OF A TEMPERATURE OF ABOUT 1040* C. FOR ABOUT 1 HOUR, GRINDING AND COMPACTING THE PRESINTERED MIXTURE UNDER PRESSURE TO FORM A SHAPED CORE, SINTERING THE SOSHAPED CORE AT A TEMPERATURE OF FROM ABOUT 1000* C. TO 1400* C. FOR ABOUT 1/4 TO 24 HOURS, AND DISCONTINUING THE SINTERING WHEN THE TEMPERATURE COEFFICIENT OF PERMEABILITY FROM ABOUT 20 * C. TO 85* C. REACHES A VALUE OF ABOUT ZERO. 